Enhanced surface passivation of quantum dots and their application in optoelectronic devices양자점 표면의 개선된 passivation 및 이의 광전자 소자에 적용

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In QDs, three-dimensional quantum confinement of excitons leads to their size-dependent electronic and optical properties which deviate significantly from those of bulk materials. The unique optical properties, including a tunable emission from UV to IR, make QDs attractive in various optoelectronic applications. How-ever, further improvements in device performance are required to make them more advanced. The well-known factor that presently limits the performance of QD thin film devices is mid-band-gap states, also known as trap states. This trap states have detrimental effect on optical properties and device performance by providing alter-native pathways for exciton quenching and carrier recombination. As a solution to this problem, chemical modi-fication of QDs has been commonly used to passivate trap states, enhance the optical properties of QDs and promote QD-based device performance. In this thesis, I intend to discover an efficient approach to mitigate the trap states by applying a pas-sivation agent which is able to occupy the defect sites and accelerate the charge transport through the QD as-semblies in thin films without diminishing the packing density of the films. First, I assume that surface dangling bonds are acting as a major source of trap states. To mitigate the effect of dangling bonds, the QD surface modification is used through the control of solution pH via mutual effect of strong acids resulting in surface ligand exchange and elimination of dangling bonds simultaneously. The investigations were performed on InP QDs resulting in improved photoluminescence quantum yield and good colloidal stability in polar solvents without long organic ligand passivation on the surface. The next part of my work is based on the QD surface passivation by reduced graphene oxide (rGO) nanosheets (NSs) acting as surface defect passivation agents via strong interaction of polar oxygen functional groups on rGO matrix (carbonyl, hydroxyl, or epoxy groups) with the cationic defect sites on QDs. Additionally, the highly conductive rGO NSs are reported to accelerate the photogenerated charge carrier transport in QD films. For this part of research, I have focused on lead sulfide (PbS) QDs which have great potential in applica-tion of optoelectronic devices such as photovoltaics, light emitting diodes, photodetectors, etc. A facile method to prepare the nanocomposite structure of rGO NSs and PbS QD hybrids via one-pot solution synthesis is demonstrated resulting in the preserved optical properties and improved exciton lifetime characteristics. The extended exciton lifetime data indicate directly the improved surface passivation. PbS QDs are known to be treated with halogen atom passivation to achieve more perfect passivation by having access to the most hidden defects on the surface. This process is performed after QD synthesis in solution state, and resulting QDs are used in fabrication of thin films with exchanging the insulating long alkyl-chain lig-ands with small organic molecules. Through my work, I introduce a thin shell layer of cadmium sulfide (CdS) grown on the halogen passivated PbS-Cl core to advance the passivation of surface defect states. Further, the thermal treatment of Type-I core/thin-shell QD films is resulted to formation of PbS-Cl optically active core which is encapsulated in the protecting matrix of CdS acting as a physical barrier between core and external environment and improving the chemical stability of PbS-Cl QDs. The core/thin-shell architecture is also demon-strated for rGO/PbS-Cl nanocomposite hybrid structures. In the last part of my work, I demonstrate the prepared core and core/thin-shell QDs with and without chemical grafting on rGO NSs into the depleted heterojunction solar cell structure to explore the influence of surface passivation on device performance. The application of core/thin-shell structured QD-based cells show increased values of open circuit voltage $(V_{oc})$, however compromises in short circuit current $(J_{sc})$ values due to the higher energy barrier of CdS-shell. The bilayer structure of photovoltaic cell is reported involving the similar thicknesses of core/thin-shell QD film with only-core QD film intending to increase the photogenerated current density. The cells which are based on the combination of halogen treated rGO/PbS-Cl only-core nanocomposite are presented to show the highest device performance measured at inert atmosphere. Finally, I present the inves-tigation on the stability of fabricated devices against the oxygen and moisture from the environment. This gives a complementary information to current-voltage measurements that are widely used for characterization meas-urements, to reveal the efficient model of passivation of QDs showing the highly stable device features of halo-gen treated rGO/PbS-Cl / CdS core/thin-shell nanocomposites.
Advisors
Jeon, Duk Youngresearcher전덕영researcher
Description
한국과학기술원 :신소재공학과,
Publisher
한국과학기술원
Issue Date
2016
Identifier
325007
Language
eng
Description

학위논문(박사) - 한국과학기술원 : 신소재공학과, 2016.2 ,[X, 99 p. :]

Keywords

Quantum Dots (QDs); Surface passivation; Core/shell QDs; Carrier transport; rGO/QD nanocomposite; QD-based solar cell; 양자점; 표면 보호; 코어 / 쉘 양자점; 캐리어 수송; rGO / 양자점 나노 복합체; QD 기반의 태양 전지

URI
http://hdl.handle.net/10203/222229
Link
http://library.kaist.ac.kr/search/detail/view.do?bibCtrlNo=648184&flag=dissertation
Appears in Collection
MS-Theses_Ph.D.(박사논문)
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